1. Field of the Invention
This invention relates to the qualification of master patterns as used in a patterning system, wherein the master pattern is qualified in-situ, and the detected defects are a measure of the quality of said master pattern. In particular, this invention relates to the qualification of a reticle used in the photolithographic patterning processes in the manufacture of integrated circuits, wherein the qualification method utilizes a resist coated quartz substrate and a transmitted light automatic inspection system.
2. Description of the Prior Art
Methods for the qualification of master patterns used in patterning systems have been in use for a number of years and have been described in the literature for some time. Furthermore, equipment and various objects designed or modified for use with these processes have also been commercially available.
There are three basic techniques that have been used: (1) carefully inspect the master pattern before it is installed in the patterning system, or provide a device which inspects it while it is in the patterning system, (2) pattern one or a small number of product objects and then inspect them carefully before subsequent product objects are patterned, or, (3) use a substitute object which is patterned and then is carefully inspected prior to patterning of product objects.
Referring to the first method described above, wherein the master pattern is inspected prior to use and certified to be defect free, it is possible that the master pattern may become contaminated or damaged while it is being moved from the inspection equipment to the patterning system and may receive further contamination or damage while it is being loaded in the patterning system. In addition, the master pattern may become contaminated while the patterning system is operating or it may become defective due to degradation of the pattern material through natural causes such as flaking or electrostatic damage. These problems will generally go undetected until the master pattern is unloaded and reinspected, this may allow significant product to be patterned with a defective master, thus reducing process yield. An improvement to this technique utilizes an inspection apparatus located in the patterning system.
In the case of wafer steppers used for photolithographic patterning of integrated circuits, a laser scanner has been available from one manufacturer as described in the data sheet from Nikon Precision, Inc., describing the model NSR-1505G2A "Step and Repeat System"; this device has been of limited usefulness since it responds only to particle type contamination which has sufficient height to scatter a laser beam, it has no capability to detect flaking of the pattern material, scratches, nor particles with low height but wide extent. In recent years, pellicles have been in use to cover the surfaces of the master pattern to further prevent contamination, although problems with contamination and ESD continue to be reported in the literature, refer to the article "Glass Wafer Processing and Inspection for Qualification of Reticles in a Fineline Wafer Stepper Production Facility" R. T. Hilton, T. E. Zavecs, J. A. Reynolds, Proceedings Of SPIE--Optical Microlithography IV, SPIE Volume 538, 1985.
Referring to the second technique described above, wherein one or a small number of product is patterned and then carefully inspected prior to patterning of additional product, this technique sacrifices one or several product objects which may have already received several process steps prior to this patterning and thus this technique may be very expensive. Furthermore, this patterning step may interact with one or more of the prior steps and cause an inspection of the present pattern to be very difficult. In the case of photolithographic patterning of integrated circuit wafers, there is equipment available which can inspect one of many layers present on a wafer, but the equipment is very slow and very expensive, refer to the data sheet from KLA Instruments describing the model KLA-2020 Wafer Inspector dated 1984. Oftentimes the master pattern is changed frequently requiring frequent qualification. Furthermore, in the patterning of integrated circuits, it has been observed that although a master pattern may stay in the patterning system for an extended period, it is necessary to frequently re-qualify it in order to detect contamination or pattern material damage that may have occurred since the last qualification. Since patterning of product wafers is generally halted until the qualification step has been completed, it is desirable to have a very fast qualification process.
The third technique described above uses a substitute object for qualification which closely resembles the product object. In this technique, prior to patterning of the product objects, the substitute or monitor object is patterned and then inspected. The selection of the monitor object is important to the qualification process, in that it should respond to the patterning process as closely to a product object as possible and yet is should be able to be inspected by available inspection equipment easily, quickly and with high quality. The degree to which these criterion are met determines the effectiveness of the qualification process.
For the qualification of photolithographic patterning equipment used in the manufacture of integrated circuits, a common technique has been to use inspection equipment designed for the automatic inspection of photomasks and reticles as described in the abovementioned article by R. T. Hilton et al. This equipment, as described in the data sheet from KLA Instruments describing the model KLA-229 "Automatic Reticle and Photomask Inspection System", has the advantage of being readily available, operates at high inspection speeds, and is of reasonable cost. Since this type of inspection machine is designed to inspect photomasks and reticles, the monitor object used for the qualification of the photolithographic system must have characteristics like a photomask or reticle before it can be inspected, i.e. a transparent glass or quartz substrate with a very opaque, very thin coating such as chrome which contains the pattern to be inspected. In addition, the monitor object must be able to be handled and patterned by the patterning system, thus requiring it to have the same physical dimensions, resist coating, and opacity to infra-red illumination as does a silicon wafer. The opacity to infra-red illumination is important since many photolithographic patterning systems use infra-red sensors to detect the presence, position and orientation of wafers as well as use infra-red illumination for autofocussing purposes.
This prior art technique, then, uses a transparent glass or quartz substrate of the same physical dimensions as a silicon wafer; the aforementioned substrate is then coated with a thin layer of chrome and then coated with the resist used by the patterning system--this is called a chrome glass wafer. The patterning system handles this monitor object just like a product wafer--the chrome coating provides sufficient infra-red opacity, the monitor is the same physical size for proper handling, and the resist coating responds to the master pattern for proper patterning. The patterned monitor is then developed, the chrome etched, and the resist stripped. The monitor object now has the target pattern from the patterning system contained in the chrome coating, it has the same optical characteristics as a photomask or reticle, and needs only a holder to adapt its physical size to the inspection system; these holders are readily available from most inspection equipment manufacturers including KLA Instruments.
This patterning qualification technique has been in use for a number of years and avoids all of the disadvantages mentioned earlier for the other two techniques. This technique, however, has the disadvantages that it requires a separate chrome etching step for which equipment is not available in a wafer fabrication area and must be purchased separately. Furthermore, great care must be exercised in the etching step to accurately etch the chrome to reproduce the resist pattern so as to yield an accurate representation of the master pattern. In addition, the chrome glass wafers are expensive and are not reuseable which further adds to the cost, and finally, chrome is a heavy metal which is known to contaminate silicon wafers and must be carefully handled--in particular, recent evidence has suggested that slight amounts of chrome are left deposited on the wafer track equipment and the chuck of the stepper insomuch that the chrome particles contaminate subsequent silicon wafers. Experimentation with other metal coatings such as aluminum and titanium have been reported. These coatings could avoid the contamination problem.
Another variant of the monitor object approach uses a normal blank silicon wafer (no prior processing) as the monitor substrate and coats it as normal with resist. This monitor wafer is then patterned, developed and inspected with a reflected light inspection system such has become available in recent years, and is described in the data sheet from Sony Corporation which describes the model ARQUS-20 "Automatic Reticle Qualification System", and in the data sheet from NJS Corporation describing the model 3WD36 "Automatic Wafer Inspection System". This prior art method has the advantage that there are no extra processing steps, no heavy metal contamination, and the monitor wafers are reuseable, however, the inspection equipment that has been available to date has been very slow and expensive and have yielded only marginal inspection performance. This is partly due to the low light levels and low contrast typical of reflected light imaging of resist coated silicon. A number of techniques have been used recently to improve the contrast of the resist coated areas of the pattern versus the non-resist coated areas such as using dyes in the resist or using spectral filters in the illumination of the inspection station. These techniques have yielded somewhat better results.